1. Field of Invention
The present invention relates to an improved system for processing reflected energy signals such as laser light pulses. More specifically, the invention concerns a system for using reflected energy signals to identify flat, orthogonal surfaces.
2. Description of Related Art
Many different systems are known for acquiring electrical images representative of visual scenes such as landscapes, buildings, military targets, and the like. "Passive" systems are one example of such systems. In passive systems, a detector is used to sense energy produced or reflected from the objects in the scene of interest. One example of a passive system is an infrared sensor that detects heat produced by objects. Alternatively, a light sensor such as an array of photodiodes may be used to sense the ambient light reflected by the objects in the scene. In contrast to passive systems, "active" systems actively generate energy, reflect energy off of the objects, and detect the reflected energy signals.
Passive systems have been utilized widely for quite some time, and many users have found them to be satisfactory for certain applications. However, when considered in other applications, passive systems have been found to have a number of limitations. First of all, it may be difficult to interpret the data provided by passive systems, since a given level of light intensity may indicate an object with low reflectivity, or the same intensity level may indicate a distant object with high reflectivity.
As a result, many users have turned to laser detecting and ranging (LADAR) systems, which employ light detecting equipment to record light signals generated by a laser light source and reflected by one or more objects. Typically, a laser light source transmits laser light signals toward a scene in a pattern such as a raster, star sequence, or circle sequence. Light detecting equipment, which is often co-located with the laser light source, detects light signals that are reflected by the objects and terrain of the scene. With LADAR systems, the range of a distant object is derived from the time the laser light travels to and from the object. Moreover, information related to the hue, orientation, texture, reflectivity, and shape of a distant object may be derived from the intensity of light reflected by the object.
Data collected by a LADAR system is typically processed by an electronic device such as a computer. When LADAR systems are used in targeting applications, such as missile guidance operations, one of the chief goals of such processing is to accurately identify targets, and distinguish them from non-targets. Therefore, engineers involved in the research and development of LADAR-based targeting systems are constantly seeking to improve the ability of such systems to accurately recognize targets.
One approach that has been used with LADAR systems to recognize targets is the "slope-based" technique. In one example of a slope-based technique, a computer identifies flat surfaces by mathematically fitting planes to data points representative of the surfaces. The least squares method is one known means for accomplishing such a fit.
Although the slope-based technique is useful in a number of respects, it is limited in some other ways. In particular, the slope-based technique may be more noise sensitive than some users desire. A problem arises if the slope-based technique is applied to data points that represent a flat surface, but contain one noisy data point that differs substantially from the other points. In this case, the noisy point will skew the computer's attempts to fit a plane to the data points. This is especially true if the least squares fitting method is used, since the difference between the squares of the data points and the square of the noisy point is magnified.